The invention relates to methods and apparatus for the provision of ventilatory assistance matched to a subject""s respiratory need. The ventilatory assistance can be for a subject who is either spontaneously or non-spontaneously breathing, or moves between these breathing states. The invention is especially suitable for, but not limited to, spontaneously breathing human subjects requiring longterm ventilatory assistance, particularly during sleep.
Subjects with severe lung disease, chest wall disease, neuromuscular disease, or diseases of respiratory control may require in-hospital mechanical ventilatory assistance, followed by longterm home mechanical ventilatory assistance, particularly during sleep. The ventilator delivers air or air enriched with oxygen to the subject, via an interface such as a nosemask, at a pressure that is higher during inspiration and lower during expiration.
In the awake state, and while waiting to go to sleep, the subject""s ventilatory pattern is variable in rate and depth. Most known ventilatory devices do not accurately match the amplitude and phase of mask pressure to the subject""s spontaneous efforts, leading to discomfort or panic. Larger amounts of asynchrony also reduce the efficiency of the device. During sleep, there are changes in the neural control of breathing as well as the mechanics of the subject""s airways, respiratory muscles and chest wall, leading to a need for substantially increased ventilatory support. Therefore, unless the device can automatically adjust the degree of support, the amplitude of delivered pressure will either be inadequate during sleep, or must be excessive in the awake state. This is particularly important in subjects with abnormalities of respiratory control, for example central hypoventilation syndromes, such as Obesity Hypoventilation Syndrome, where there is inadequate chemoreceptor drive, or Cheyne Stokes breathing such as in patients with severe cardiac failure or after a stroke, where there is excessive or unstable chemoreceptor device.
Furthermore, during sleep there are inevitably large leaks between mask and subject, or at the subject""s mouth if this is left free. Such leaks worsen the error in matching the phase and magnitude of the machine""s effort to the subject""s needs, and, in the case of mouth leak, reduce the effectiveness of the ventilatory support.
Ideally a ventilatory assistance device should simultaneously address the following goals:
(i) While the subject is awake and making substantial ventilatory efforts, the delivered assistance should be closely matched in phase with the patient""s efforts.
(ii) The machine should automatically adjust the degree of assistance to maintain at least a specified minimum ventilation, without relying on the integrity of the subject""s chemoreflexes.
(iii) It should continue to work correctly in the presence of large leaks.
Most simple home ventilatory either deliver a fixed volume, or cycle between two fixed pressures. They do so either at a fixed rate, or are triggered by the patient""s spontaneous efforts, or both. All such simple devices fail to meet goal (ii) of adjusting the degree of assistance to maintain at least a given ventilation. They also largely fail to meet goal (i) of closely matching the subjects respiratory phase: timed devices make no attempt to synchronize with the subject""s efforts; triggered devices attempt to synchronize the start and end of the breath with the subject""s efforts, but make no attempt to tailor the instantaneous pressure during a breath to the subject""s efforts. Furthermore, the triggering tends to fail in the presence of leaks, thus failing goal (iii).
The broad family of servo-ventilators known for at least 20 years measure ventilation and adjust the degree of assistance to maintain ventilation at or above a specified level, thus meeting goal (ii), but they still fail to meet goal (i) of closely matching the phase of the subject""s spontaneous efforts, for the reasons given above. No attempt is made to meet goal (iii).
Proportional assistist ventilation (PAV), as taught by Dr Magdy Younes, for example in Principles and Practice of Mechanical Ventilation, chapter 15, aims to tailor the pressure vs time profile within a breath to partially or completely unload the subject""s resistive and elastic work, while minimizing the airway pressure required to achieve the desired ventilation. During the inspiration half-cycle, the administered pressure takes the form:
P(t)=P0+R.fRESP(t)+E.V(t) 
where R is a percentage of the resistance of the airway, fRESP(t) is the instantaneous respiratory airflow at time t, E is a percentage of the elastance of lung and chest wall, and V(t) is the volume inspired since the start of inspiration to the present moment. During the expiratory half-cycle, V(t) is taken as zero, to produce passive expiration.
An advantage of proportional assist ventilation during spontaneous breathing is that the degree of assistance is automatically adjusted to suit the subject""s immediate needs and their pattern of breathing, and is therefore comfortable in the spontaneously breathing subject. However, there are at least two important disadvantages. Firstly, V(t) is calculated as the integral of flow with respect to time since the start of inspiration. A disadvantage of calculating V(t) in this way is that, in the presence of leaks, the integral of the flow through the leak will be included in V(t), resulting in an overestimation of V(t), in turn resulting in a runaway increase in the administered pressure. This can be distressing to the subject. Secondly, PAV relies on the subject""s chemoreceptor reflexes to monitor the composition of the arterial blood, and thereby set the level of spontaneous effort. The PAV device then amplifies this spontaneous effort. In subject with abnormal chemoreceptor reflexes, the spontaneous efforts may either cease entirely, or become unrelated to the composition of the arterial blood, and amplification of these efforts will yield inadequate ventilation. In patients with existing Cheyne Strokes breathing during sleep. PAV will be design amplify the subject""s waxing and waning breathing efforts, and actually make matters worse by exaggerating the disturbance. Thus PAV substantially meets goal (i) of providing assistance in phase with the subject""s spontaneous ventilation, but cannot meet goal (ii) of adjusting the depth of assistance if the subject has inadequate chemoreflexes, and does not satisfactorily meet goal (iii).
Thus there are known devices that meet each of the above goals, but there is to device that meets all the goals simultaneously. Additionally, it is desirable to provide improvements over the prior art directed to any one of the stated goals.
Therefore, the present invention seeks to achieve, at least partially, one or more of the following:
(i) to match the phase and degree of assistance to the subject""s spontaneous efforts when ventilation is well above a target ventilation,
(ii) to automatically adjust the degree of assistance to maintain at least a specified minimum average ventilation without relying on the integrity of the subject""s chemoreflexes and to damp out instabilities in the spontaneous ventilatory efforts, such as Cheyne Stokes breathing.
(iii) to provide some immunity to the effects of sudden leaks.
In what follows, a fuzzy membership function is taken as returning a value between zero and unity, fuzzy intersection A AND B is the smaller of A and B, fuzzy union A OR B is the larger of A and B, and fuzzy negation NOT A is 1xe2x88x92A.
The invention discloses the determination of the instantaneous phase in the respiratory cycle as a continuous variable.
The invention further discloses a method for calculating that instantaneous phase in the respiratory cycle including at least the steps of determining that if the instantaneous airflow is small and increasing fast, then it is close to start of inspiration, if the instantaneous airflow is large and steady, then it is close to mid-inspiration, if the instantaneous airflow is small and decreasing fast, then it is close to mid-expiration, if the instantaneous airflow is zero and steady, then it is during an end-expiratory pause, and airflow conditions intermediate between the above are associated with correspondingly intermediate phases.
The invention further discloses a method for determining the instantaneous phase in the respiratory cycle as a continuous variable from 0 to 1 revolution, the method comprising the steps of:
selecting at least two identifiable features FN of a prototype flow-vs-time waveform f(t) similar to an expected respiratory flow-vs-time waveform, and for each said feature:
determining by inspection the phase xcfx86N in the respiratory cycle for said feature, assigning a weight WN to said phase,
defining a xe2x80x9cmagnitudexe2x80x9d fuzzy set MN whose membership function is a function of respiratory airflow, and a xe2x80x9crate of changexe2x80x9d fuzzy set CN, whose membership function is a function of the time derivative of respiratory airflow, chosen such that the fuzzy intersection MN AND CN will be larger for points on the generalized prototype respiratory waveform whose phase is closer to the said feature FN than for points closer to all other selected features,
setting the fuzzy inference rule RN for the selected feature FN to be: If flow if MN and rate of change of flow is CN then phase=xcfx86N, with weight WN.
measuring leak-corrected respiratory airflow,
for each feature FN calculating fuzzy membership in fuzzy sets MN and CN,
for each feature FN applying fuzzy inference rule RN to determine the fuzzy extent YN=MN AND CN to which the phase if xcfx86N, and
applying a defuzzification procedure using YN at phases xcfx86N and weights WN to determine the instantaneous phase xcfx86.
Preferably, the identifiable features include zero crossings, peaks, inflection points or plateaus of the prototype flow-vs-time waveform. Furthermore, said weights can be unity, or chosen to reflect the anticipated reliability of deduction of the particular feature.
The invention further discloses a method for calculating instantaneous phase in the respiratory cycle as a continuous variable, as described above, in which the step of calculating respiratory airflow includes a low pass filter step to reduce non-respiratory noise, in which the time constant of the low pass filter is an increasing function of an estimate of the length of the respiratory cycle.
The invention further discloses a method for measuring the instantaneous phase in the respiratory cycle as a continuous variable as described above, in which the defuzzification step includes a correction for any phase delay introduced in the step of low pass filtering respiratory airflow.
The invention further discloses a method for measuring the average respiratory rate, comprising the steps of:
measuring leak-corrected respiratory airflow;
from the respiratory airflow, calculating the instantaneous phase xcfx86 in the respiratory cycle as a continuous variable from 0 to 1 revolution, calculating the instantaneous rate of change of phase dxcfx86/dt, and
calculating the average respiratory rate by low pass filtering said instantaneous rate of change of phase dxcfx86/dt.
Preferably, the instantaneous phase is calculated by the methods described above.
The invention further discloses a method for providing ventilatory assistance in a spontaneously breathing subject, comprising the steps, performed at repeated sampling intervals, of:
ascribing a desired waveform template function Π(xcfx86), with domain 0 to 1 revolution and range 0 to 1,
calculating the instantaneous phase xcfx86 in the respiratory cycle as a continuous variable from 0 to 1 revolution,
selecting a desired pressure modulation amplitude A,
calculating a desired instantaneous delivery pressure as an end expiratory pressure plus the desired pressure modulation amplitude A multiplied by the value of the waveform template function Π(xcfx86) at the said calculated phase xcfx86, and
setting delivered pressure to subject to the desired delivery pressure.
The invention further discloses a method for providing ventilatory assistance in a spontaneously breathing subject as described above, in which the step of selecting a desired pressure modulation amplitude is a fixed amplitude.
The invention further discloses a method for providing ventilatory assistance in a spontaneously breathing subject as described above, in which the step of selecting a desired pressure modulation amplitude in which said amplitude is equal to an elastance multiplied by an estimate of the subject""s tidal volume.
The invention further discloses a method for providing ventilatory assistance in a spontaneously breathing subject as described above, in which the step of selecting a desired pressure modulation amplitude comprises the substeps of:
specifying a typical respiratory rate giving a typical cycle time,
specifying a present pressure modulation amplitude to apply at said typical respiratory rate,
calculating the observed respiratory rate giving an observed cycle time, and
calculating the desired amplitude of pressure modulation as said preset pressure modulation amplitude multiplied by said observed cycle time divided by the said specified cycle time.
The invention further discloses a method for providing ventilatory assistance in a spontaneously breathing subject, including at least the step of determining the extent that the subject is adequately ventilated, to said extent the phase in the respiratory cycle is determined from the subject""s respiratory airflow, but to the extent that the subject""s ventilation is inadequate, the phase in the respiratory cycle is assumed to increase at a pre-set rate, and setting mask pressure as a function of said phase.
The invention further discloses a method for providing ventilatory assistance in a spontaneously breathing subject, comprising the steps of: measuring respiratory airflow, determining the extent to which the instantaneous phase in the respiratory cycle can be determined from said airflow, to said extent determining said phase from said airflow but to the extent that the phase in the respiratory cycle cannot be accurately determined, the phase is assumed to increase at a preset rate, and delivering pressure as a function of said phase.
The invention further discloses a method for calculating the instantaneous inspired volume of a subject, operable substantially without run-away under conditions of suddenly changing leak, the method comprising the steps of:
determining respiratory airflow approximately corrected for leak,
calculating an index J varying from 0 to 1 equal to the fuzzy extent to which said corrected respiratory airflow is large positive for longer than expected, or large negative for longer than expected.
identifying the start of inspiration, and
calculating the instantaneous inspired volume as the integral of said corrected respiratory airflow multiplied by the fuzzy negation of said index J with respect to time, from start of inspiration.
The invention further discloses a method xe2x80x9cAxe2x80x9d for providing ventilatory assistance in a spontaneously breathing subject, the method comprising the steps, performed at repeated sampling intervals, of:
determining respiratory airflow approximately corrected for leak,
calculating an index J varying from 0 to 1 equal to the fuzzy extent to which said respiratory airflow is large positive for longer than expected, or large negative for longer than expected,
calculating a modified airflow equal to said respiratory airflow multiplied by the fuzzy negation of said index J,
identifying the phase in the respiratory cycle,
calculating the instantaneous inspired volume as the integral of said modified airflow with respect to time, with the integral held at zero during the expiratory portion of the respiratory cycle,
calculating a desired instantaneous delivery pressure as a function at least of the said instantaneous inspired volume, and
setting delivered pressure to subject to the desired delivery pressure.
The invention further discloses a method xe2x80x9cBxe2x80x9d for providing ventilatory assistance in a spontaneously breathing subject, comprising the steps of:
determining respiratory airflow approximately corrected for leak,
calculating an index J varying from 0 to 1 equal to the fuzzy extent to which the respiratory airflow is large positive for longer than expected, or large negative for longer than expected,
identifying the phase in the respiratory cycle,
calculating a modified respiratory airflow equal to the respiratory airflow multiplied by the fuzzy negation of said index J.
calculating the instantaneous inspired volume as the integral of the modified airflow with respect to time, with the integral held at zero during the expiratory portion of the respiratory cycle,
calculating the desired instantaneous delivery pressure as an expiratory pressure plus a resistance multiplied by the instantaneous respiratory airflow plus a nonlinear resistance multiplied by the respiratory airflow multiplied by the absolute value of the respiratory airflow plus an elastance multiplied by the said adjusted instantaneous inspired volume, and
setting delivered pressure to subject to the desired delivery pressure.
The invention yet further discloses a method xe2x80x9cCxe2x80x9d for providing assisted ventilation to match the subject""s need, comprising the steps of:
describing a desired waveform template function Π(xcfx86), with domain 0 to 1 revolution and range 0 to 1,
determining respiratory airflow approximately corrected for leak,
calculating an index J varying from 0 to 1 equal to the fuzzy extent to which the respiratory airflow is large positive for longer than expected, or large negative for longer than expected,
calculating JPEAK equal to the recent peak of the index J,
calculating the instantaneous phase in the respiratory cycle,
calculating a desired amplitude of pressure modulation, chosen to servo-control the degree of ventilation to at least exceed a specified ventilation,
calculating a desired delivery pressure as an end expiratory pressure plus the calculated pressure modulation amplitude A multiplied by the value of the waveform template function Π(xcfx86) at the said calculated phase xcfx86, and
setting delivered pressure to subject to said desired instantaneous delivered pressure.
The invention yet further discloses a method for providing assisted ventilation to match the subject""s need, as described above, in which the step of calculating a desired amplitude of pressure modulation, chosen to servo-control the degree of ventilation to at least exceed a specified ventilation, comprises the steps of:
calculating a target airflow equal to twice the target ventilation divided by the target respiratory rate,
deriving an error term equal to the absolute value of the instantaneous low pass filtered respiratory airflow minus the target airflow, and
calculating the amplitude of pressure modulation as the integral of the error term multiplied by a gain, with the integral clipped to lie between zero and a maximum.
The invention yet further discloses a method for providing assisted ventilation to match the subject""s need, as described above, in which the step of calculating a desired amplitude of pressure modulation, chosen to servo-control the degree of ventilation to at least exceed a specified ventilation, comprises the following steps:
calculating a target airflow equal to twice the target ventilation divided by the target respiratory rate.
deriving an error term equal to the absolute value of the instantaneous low pass filter respiratory airflow minus the target airflow,
calculating an uncorrected amplitude of pressure modulation as the integral of the error term multiplied by a gain, with the integral clipped to lie between zero and a maximum.
calculating the recent average of said amplitude as the low pass filtered amplitude, with a time constant of several times the length of a respiratory cycle, and
setting the actual amplitude of pressure modulation to equal the said low pass filtered amplitude multiplied by the recent peak jamming index JPEAK plus the uncorrected amplitude multiplied by the fuzzy negation of JPEAK.
The invention yet further discloses a method for providing assisted ventilation to match the subject""s need, and with particular application to subjects with varying respiratory mechanics, insufficient respiratory drive, abnormal chemoreceptor reflexes, hypoventilation syndromes, or Cheyne Stokes breathing, combined with the advantages of proportional assist ventilation adjusted for sudden changes in leak, comprising the steps, performed at repeated sampling intervals, of:
calculating the instantaneous mask pressure as described for methods xe2x80x9cAxe2x80x9d or xe2x80x9cBxe2x80x9d above,
calculating the instantaneous mask pressure as described for method xe2x80x9cCxe2x80x9d above,
calculating a weighted average of the above two pressures, and
setting the mask pressure to the said weighted average.
The invention yet further discloses apparatus to give effect to each one of the methods defined, including one or more transducer to measure flow and/or pressure, processor means to perform calculations and procedures, flow generators for the supply of breathable gas at a pressure above atmospheric pressure and gas delivery means to deliver the breathable gas to a subject""s airways.
The apparatus can include ventilators, ventilatory assist devices, and CPAP devices including constant level, bi-level or autosetting level devices.
It is to be understood that while the algorithms embodying the invention are explained in terms of fuzzy logic, approximations to these algorithms can be constructed without the use of the fuzzy logic formalism.